1. Field of the Invention
The invention comprises a random wavelength access monochromator incorporating a pair of coaxial off-axis parabolic reflectors whose spherical aberrations mutually cancel each other.
2. Description of Related Art
For close to a century, optical instrument manufacturers have been able to produce devices capable of dividing a broad band beam of light into individual wavelengths, and emitting only desired wavelength(s). These devices could be manually changed to emit different wavelength bands.
More recently, motor drives have been added that allowed rapid and automatic changes between wavelength bands. The speed of transition from one wavelength band to another has been, and is still, limited by the torque generating capacity of the motor, and the mass of the mechanism used to divide the broad band light into its discrete wavelength bands. High precision stepping motors are typically used, with large ratio (200:1) reduction-gear attached to provide the necessary precision of positioning. Typical positioning requirements are on the order of 0.004 degrees, equivalent to a change of 0.25 nm of optical wavelength for a 1/4 meter class monochromator (assuming 1200 l/mm grating).
While these motors do provide for improved usefulness and control over manual methods, their speed is limited by the fact that they must always move in increments of precision equivalent to their smallest step, and they must always move using the same high ratio reduction gear drive. Therefore, to move from a position of, for example, 450 nm to 600 nm, if the system had a precision of 0.25 nm, the motor has to make 600 individual steps. Further, each step is driving a 200:1 drive train. As a function of physical laws, the greater the speed of the transition, the less accurate the positioning. Conversely, the more accurate the positioning, the slower the transition. Typical practical transition times are "2100 nm per minute", or approximately 30 milliseconds per nm. The transition time for the example above would be 30 milliseconds.times.150 nm=4.5 seconds with an accuracy of 1 nm.
In order to achieve a more rapid transition between wavelength bands, the use of multiple beams of filtered wavelengths, combined by optical-mechanical methods, is prevalent. This is a costly solution, and does not provide for sequential wavelength scanning of the full spectrum of wavelengths.
The recent development of high speed, low torque oscillating motors, or galvanometer scanners has improved the situation somewhat. The scanner is typically attached to a small mirror, which oscillates between two angular positions. When properly positioned near the exit aperture of the monochromator, the instrument can oscillate between two discrete wavelength bands of emitted light. Unfortunately, these motors are not capable of producing very high torque. Therefore, due to mass constraints, the mirror must be relatively small. The small size of the mirror severely restricts the optical wavelength range within which the instrument can oscillate (usually 50-100 nm), which means that the instrument must have a second (slower, higher precision) drive to extend the oscillating range to a more practical range. Another problem with this methodology is that the beam exiting the monochromator changes angle as it changes wavelength. The closer the scanning mirror is to the exit slit, and the greater the transition angle of the scanner, the greater the angle that the beam changes. This leads to alignment problems with "downstream" optics. Further, the scanners can typically only oscillate at fixed resonant frequencies about 2 fixed positions. This prevents the instrument from being useful for anything but slewing or oscillation between fixed wavelengths, at fixed separations, at fixed speeds. There is no practical way to scan or randomly select wavelengths at various high speeds using this methodology.
More recent developments in scanner motor design have led to a scanner that can not only oscillate, but can move randomly or sequentially throughout its full angular transition range. The precision of positioning is on the order of 0.004.degree. within that range. At least one instrument has been developed that uses this class of motor to select emitted wavelengths. This instrument directly couples the motor to the optical component that separates the broad band light into discrete wavelengths by using a moving grating. However, due to the mass of the grating and its mount, the transition speed is limited. The transition time between wavelengths is only in the millisecond time domain when the wavelengths are relatively close (50 nm), and only when moving in one direction. The inertia of the same component, and the low torque capacity of the motor, requires additional time to slow, stop, and accelerate the component back to the original position. This restriction prohibits the instrument from being able to maintain millisecond transition times for almost any practical wavelength separation range, or mode of change.
The prior art patent literature describes a variety of devices that use multiple mirrors, some of them parabolic, in the context of slit imaging systems. A relevant sampling of those patents is described below.
U.S. Pat. No. 4,634,276 entitled SLIT IMAGING SYSTEM USING TWO CONCAVE MIRRORS describes an off-axis system employing two mirrors P.sub.1 and P.sub.2.
U.S. Pat. No. 5,532,818 entitled DIFFERENCE DISPERSIVE DOUBLE PATH MONOCHROMATOR HAVING A WAVELENGTH INDEPENDENT IMAGING POINT describes a monochromator that employs two parabolic mirrors to reflect the path of the light energy.
U.S. Pat. No. 5,089,915 entitled FABRICATION OF ASPHERIC SURFACES THROUGH CONTROLLED DEFORMATION OF THE FIGURE OF SPHERICAL REFLECTIVE SURFACES describes another system which could employ an off-axis parabolic mirror or a torque mirror in a specific context.
U.S. Pat. No. 4,995,725 entitled MONOCHROMATOR ARRANGEMENT also describes a monochromator system employing a parabolic mirror.
U.S. Pat. No. 5,384,656 entitled ASTIGMATISM CORRECTED GRATINGS FOR PLANE GRATING AND SPHERICAL MIRROR SPECTROGRAPHS describes another arrangement for correcting astigmatic problems.
U.S. Pat. No. 5,305,083 entitled RANDOM ACCESS MONOCHROMATOR suggests the general state of the art.
U.S. Pat. No. 5,192,981 describes a CZERNY-TURNER MONOCHROMATOR which is typical of early prior art efforts and is a technique that is generally well known.
U.S. Pat. No. 5,497,231 entitled MONOCHROMATOR HAVING AN OSCILLATING MIRROR BEAM-DIFFRACTING ELEMENT FOR SPECTROMETERS describes another technique employing lenses, a two position oscillatory mirror, and a grating.
The following patents are cited as being of general interest only and do not appear to be otherwise relevant to the specific invention: U.S. Pat. Nos. 4,984,888; 4,995,721; 5,448,351; and 5,285,255.
In general, insofar as understood, none of the prior art when taken individually or in combination appears to suggest the novel system described herein in which two concave parabolic mirrors are aligned in a coaxial arrangement such that their spherical aberrations subtract and substantially cancel. Moreover, this arrangement is preserved by the use of a folding mirror located between the dispersive elements, i.e., the grating, and either one of the concave off-axis parabolic mirrors. Lastly, the invention is unusual in the fact that the same folding mirror, and not the grating (as is typically the case), is used as the primary means of a wavelength selected.